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Step 5General info about 'Recapping'
ALL tube amps need a recapping job at some point. Old power supply 'filter' capacitors are the primary cause of hum. And used or not, the capacitors fail over time. Electrolytic (tubular) capacitors have an optimum life of 10 years. Of course, they may last much longer. They will often operate sub-optimally for much longer.
But tube gear is sensitive to cap failure. Tubes circuits operate at higher voltages, and many amps are quite simple. Older caps with outmoded technology ('paper electrolytic') are almost certain to fail over time. In one sense, tubes are less susceptible to cap failure--unlike solid-state amps, many will continue to operate, although poorly.
Cap types
Most common replacement caps are:
Non-polarized (polypropylene, or mylar)
Rating: 500V minimum
Most builders (and rebuilders) use high quality polypropylene ('orange drop') caps. But a quality mylar cap exceeds the characteristics of the original capacitors. I wanted to retain the vintage sound, so I choose mylar. It's amateur builders, not pros, that insist a certain type of cap is a must. In any event, the difference is subtle, and the qualitative differences are a matter of personal taste.
Is the difference between cap types real? Probably.
Here's an interesting cap bench test.
Polarized (electrolytic)
Rating: depends on the amp. I used 350V caps where the originals were 150V.
Regular 'computer' type electrolytic capacitors work fine. Especially for the power supply filter caps.
They come in two packages: radial, with the leads on the same end, and axial, with the leads on inline on both ends.
Oh, Yeah...as the name implies, polarized caps must be installed with the correct....polarity. Inserting one backwards--fireworks!
Note: The first set of filter caps installed were purchased at a local electronics store. This store is nerd heaven--or so I thought. Some of their components are in yellowing packaging; clearly getting old. I picked what looked like the newest of the bunch. The caps (40uF, 350V) lasted for two weeks before failing--first a bit of crackling, then a lot of crackling and the amp began cutting out....
From now on, it's mail-order from a reputable firm (digikey, jameco, etc.)
But tube gear is sensitive to cap failure. Tubes circuits operate at higher voltages, and many amps are quite simple. Older caps with outmoded technology ('paper electrolytic') are almost certain to fail over time. In one sense, tubes are less susceptible to cap failure--unlike solid-state amps, many will continue to operate, although poorly.
Cap types
Most common replacement caps are:
Non-polarized (polypropylene, or mylar)
Rating: 500V minimum
Most builders (and rebuilders) use high quality polypropylene ('orange drop') caps. But a quality mylar cap exceeds the characteristics of the original capacitors. I wanted to retain the vintage sound, so I choose mylar. It's amateur builders, not pros, that insist a certain type of cap is a must. In any event, the difference is subtle, and the qualitative differences are a matter of personal taste.
Is the difference between cap types real? Probably.
Here's an interesting cap bench test.
Polarized (electrolytic)
Rating: depends on the amp. I used 350V caps where the originals were 150V.
Regular 'computer' type electrolytic capacitors work fine. Especially for the power supply filter caps.
They come in two packages: radial, with the leads on the same end, and axial, with the leads on inline on both ends.
Oh, Yeah...as the name implies, polarized caps must be installed with the correct....polarity. Inserting one backwards--fireworks!
Note: The first set of filter caps installed were purchased at a local electronics store. This store is nerd heaven--or so I thought. Some of their components are in yellowing packaging; clearly getting old. I picked what looked like the newest of the bunch. The caps (40uF, 350V) lasted for two weeks before failing--first a bit of crackling, then a lot of crackling and the amp began cutting out....
From now on, it's mail-order from a reputable firm (digikey, jameco, etc.)
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